Coupled oscillators for tuning fluorescence properties of squaraine dyes

Combining a squaraine (S) and a BODIPY (B) chromophore in a heterodimer (SB) and two heterotrimers (BSB and SBS) by alkyne bridges leads to the formation of coupled oscillators whose fluorescence properties are superior compared to the parent squaraine chromophore. The lowest energy absorption and emission properties of these superchromophores are mainly governed by the squaraine part and are shifted by more than 1000 cm(-1) to the red by excitonic interaction between the squaraine and the BODIPY dye. Employing polarization-dependent transient absorption and fluorescence upconversion measurements, we could prove that the lowest energy absorption in SB and BSB is caused by a single excitonic state but by two for SBS. Despite the spectral red-shift of their lowest absorption band, the fluorescence quantum yields increase for SB and BSB compared to the parent squaraine chromophore SQA. This is caused by intensity borrowing from the BODIPY states, which increases the squared transition moments of the lowest energy band dramatically by 29% for SB and 63% for BSB compared to SQA. Thereby, exciton coupling leads to a substantial enhancement of fluorescence quantum yield by 26% for SB and by 46% for BSB and shifts the emission from the red into the near-infrared. In this way, the BODIPY-squaraine conjugates combine the best properties of each class of dye. Thus, exciton coupling in heterodimers and -trimers is a valuable alternative to tuning fluorescence properties by, e.g., attaching substituents to chromophores.